Chemical sensors are routinely used for the detection of various gases, chemicals, moisture, organic impurities, etc. in an ambient environment. Chemical sensors combine a chemically-sensitive interface, which sorbs chemical species (i.e., analytes) from the environment, with a physical transducer that provides an electrical output proportional to the amount of sorbed species. Typical microfabricated chemical sensors comprise a thin layer of a material that is sensitive to the concentration of a target chemical species present in the ambient atmosphere. For example, the interaction of a chemical species with the sensitive material layer can change the materials conductivity, dielectric constant, or effective thickness which can be transduced into an electrical output signal that is related to the concentration of the chemical in the ambient environment. Although many microfabricated chemical sensors exist for this purpose, a need remains for an autonomous, selective, and sensitive microfabricated chemical sensor that can be used in remote locations.
Portable, handheld microanalytical systems are also being developed to enable the rapid and sensitive detection of particular chemicals, including pollutants, high explosives, and chemical and biological warfare agents. Current gas-phase microanalytical systems typically comprise a gas chromatography column to separate the chemical species, or analytes, in a gas sample and a detector to identify the separated species. Such microanalytical systems can also include a chemical preconcentrator for sample collection. The chemical preconcentrator serves the important function of collecting and concentrating the chemical analytes on a sorptive material at the inlet of the microanalytical system. The chemical preconcentrator can deliver an extremely sharp sample plug to the downstream gas chromatograph by taking advantage of the rapid, efficient heating of the sorbed analytes with a low-heat capacity, low-loss microhotplate. The very narrow temporal plug improves separations, and therefore the signal-to-noise ratio and sensitivity to the particular chemical species of interest.
Previous microfabricated chemical preconcentrators have typically used a heated planar membrane suspended from a substrate as the microhotplate, wherein the sorptive material is disposed as a layer on a surface of the membrane to sorb the chemical species from a gas stream. The sorptive material thereby collects and concentrates the sample, and then the heated membrane thermally desorbs the sample in a short pulse for subsequent separation. See U.S. Pat. No. 6,171,378 to Manginell et al., which is incorporated herein by reference. Typically, samples are collected by the preconcentrator for a fixed period of time (e.g., 2 minutes) before they are released for analyte separation and identification. Collecting for a fixed time period is a fundamental shortcoming of the chemical analysis process. See U.S. patent application Ser. No. 10/903,329 to Manginell et al., filed Jul. 29, 2004, which is incorporated herein by reference. When concentrations of potential toxins are high, precious time is wasted collecting excess sample material. Furthermore, this excess material will often saturate the preconcentrator and overwhelm a detector, necessitating cleaning before further analysis can resume. Conversely, when target analyte concentrations in the sample stream are low, insufficient analyte may be collected for detection or proper identification. To avoid these problems, a smart, analyte-sensitive chemical preconcentrator that actively measures the change in the sorptive material's conductivity, dielectric constant, or effective thickness during the collection process is needed.
The present invention provides a microfabricated capacitive chemical sensor that can be used as an autonomous chemical sensor or as an analyte-sensitive chemical preconcentrator in a larger microanalytical system. The capacitive chemical sensor detects changes in the sensing film dielectric properties, such as the dielectric constant, conductivity, and dimensionality. These changes result from the interaction of a target analyte with the sensing dielectric film. This capability provides a low-power, self-heating chemical sensor suitable for remote and unattended sensing applications. The capacitive chemical sensor also enables a smart, analyte-sensitive chemical preconcentrator. After sorption of the sample by the sensing dielectric film, the film can be rapidly heated to release the sample for further analysis. Therefore, the capacitive chemical sensor can optimize the sample collection time prior to release to enable the rapid and accurate analysis of analytes by a microanalytical system.